<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    var num = 10

    // num = num + 1
    // ++num

    // num += 20  // num = num + 5
    // num -= 2  // num = num -2

    // num *= 20
    // num /= 20

    // num %= 6 // 4

    console.log(num)

    var num = 20, age = 18
  </script>
</body>
</html>